The present invention is directed to a new rapid tooling (RT) sintering method based on rapid prototyping (RP) technology and powder compositions therefor. Rapid tooling (RT) technologies are based on established techniques related to rapid prototyping (RP).
The formation of metal parts, especially those that are intricately or irregularly shaped such as mold cavities for the injection molding of thermoplastics, gears, sprockets, or threaded parts, may involve many process steps. Often, the formation process involves the steps of casting, machining, heat treatment for hardening, tempering, polishing and plating of the parts. Significant volume changes during casting usually require time consuming and expensive processing. Before the creation of rapid prototyping techniques, a skilled technician might take many weeks or months to machine a regular prototype model. The resulting prototype was likely to have dimensional inaccuracies due to human error.
The term xe2x80x9cprototypexe2x80x9d defines a positive model of an article and has essentially the same dimensions of compression or injection molded articles. The prototype is usually more porous than the molded article, but otherwise it is visually essentially indistinguishable from the molded article, and functions in essentially the same manner as the molded article. A prototype is used for complete evaluation of the form, design, and performance of a desired molded article.
Using a variety of technologies, it is now possible to generate prototype models in a few days or weeks. These models are termed xe2x80x9crapid prototyped modelsxe2x80x9d (RP models) and are usually based on CAD-Solid Modeling technology, which is well known in the art. xe2x80x9cCADxe2x80x9d is an abbreviation for Computer Aided Design in which the design is displayed three-dimensionally on a cathode ray tube. CAD-Solid Modeling is the method by which these three-dimensional computer models may be transformed into three dimensional solid positive models, i.e., RP models.
An example of CAD-Solid Modeling is the process of stereolithography which is well known in the art and disclosed, for example, in U.S. Pat. No. 4,575,330, issued Mar. 11, 1986 to Hull, which is hereby incorporated by reference. Generally, a photopolymerizable liquid resin is subjected to selective bombardment of radiation to form a step-wise laminar buildup of the desired object. The rapid prototype comprises a resin, which has limited strength and heat durability, but has dimensions which accurately represent the computerized data from which it was created.
With the advantages of speed and accuracy, rapid prototyping technologies advanced quickly after their introduction in the early 1980""s, and a large market has been created for rapid prototyping equipment and services. The arrival of more sophisticated solid modeling systems based on computer generated models is expected to increase the design complexity of part geometry.
The next logical step forward after the creation of rapid prototyped models is to create xe2x80x9crapid toolingxe2x80x9d (RT) which would be used to create production versions of the rapid prototype models. RT is a process of quickly and efficiently manufacturing a negative mold (xe2x80x9ctoolxe2x80x9d) from a positive RP model. By xe2x80x9cnegativexe2x80x9d it is meant that the mold will impart its form to fluid, powder or other substance and the fluid, powder or other substance adopts the dimensional form of the prototype. A xe2x80x9cpositivexe2x80x9d mold will impart its form to a fluid, powder or other substance, but a xe2x80x9cnegativexe2x80x9d reproduction of the prototype will be formed.
When the fluid, powder or other substance is stabilized or solidified, the RP can be removed leaving a tool. The tool has the negative dimensional characteristics of the prototype and can be used to form molded articles in mass production. An example of this process is injection molding. This process involves molding metal, plastic, or non-plastic ceramic shapes by injecting a measured quantity of the molten or non-polymerized material into the tool.
Ideally, rapid tools can be created in a short time and at a low cost. However, due to the limitations of strength and heat durability of RP photopolymerized resins which are made by stereolithography, most of the negative molds (tools) cannot be applied to real productivity due to warping, shrinking and melting of the resin RP when the negative mold is made. Thus, the formation of intricately or irregularly shaped parts, such as mold cavities for the injection molding of thermoplastics, gears, sprockets, or threaded parts, may involve many processing steps which are time consuming and expensive. Often, the formation process involves the steps of casting, machining, heat treatment for hardening, tempering, polishing and plating of the parts. Significant volume changes during casting may require expensive and careful processing. Machining and polishing can be particularly difficult for intricate or irregular shapes. The formation of parts from very hard, corrosion resistant alloys greatly increases the difficulty in processing.
A number of techniques have been developed to create rapid tools from rapid prototypes. The two technologies which are most popular are the Selective Laser Sintering (SLS) process and the Keltool process. The SLS process, as disclosed in U.S. Pat. No. 5,342,919, issued Aug. 30, 1994 to Dickens, Jr. et al., describes a laser-sinterable powder product which is prepared, deposited and leveled into a thin layer. Following a pattern obtained from a two dimensional section of a 3-D CAD model, a CO2 laser sinters the thin layer of the target region and generates a first slice of sintered powder in a two-dimensional shape. Subsequent layers are applied and sintered until the part is complete. The part or prototype is nearly fully dense and may be further refined by drying, firing and copper infiltration.
Thus, a rapid tool capable of running a large number of injection molded parts can be created. The turnaround time for creating production quality parts can be as little as two weeks with this process. However, the cost of this operation is reported to be very high and the accuracy of the product is still poor due to the dimensional gaps surrounding the periphery of each layer. Preparing different types of molds, such as ceramic or ceramic metal parts is also difficult. Further, because the infiltration and sintering are separate processes, some distortion and shrinkage will occur.
The Keltool process was developed more than 20 years ago. During this process a Room Temperature Vulcanizing (RTV) mold is created from a master part by pouring silicone rubber around a thermoplastic pattern, which may be based on a resin CAD-stereolithography technology. After the silicone rubber is solidified, the master part is removed from the silicone rubber by cutting the silicone rubber along a parting line. This step is difficult and time consuming. Another difficultly is the removal of the RTV mold from the green compact. Typically, the mold is heated in a furnace to burn out the silicone rubber, which causes pollution. The formed RTV mold is then filled with a metal powder/binder mixture to form a green compact which is sintered and further treated.
Another method of producing dense metal molds and parts from rapid prototyped models is disclosed in U.S. Pat. No. 5,507,336 issued Apr. 16, 1996 to Tobin. In this patent, which involves numerous steps, a resin RP pattern is placed in a steel tube and a ceramic slurry and a binder is cast around the critical surfaces thereof. The RP pattern is burned out, preferably by heating in a furnace at 1100xc2x0 F. for three hours, and metal powder is cast around the critical surfaces of the ceramic member. An infiltration metal is placed over the powder and the apparatus is placed in a furnace at 2100xc2x0 F. for at least one hour. The ceramic member is then removed and what is formed is a metal part or a mold half suitable for mating with another mold half to form a mold for casting multiple parts.
This procedure suffers from numerous draw backs. First, the final sintered metal part or mold is two degrees removed from the RP pattern. The RP pattern is transferred to the ceramic member, which is in turn transferred from the ceramic member to the metal particles. This increases the possibility of errors in the final metal product. The additional steps increase the time and expense of the process. Further, during the infiltration process the steel tube thermally expands at a greater rate than the ceramic member. This causes a gap between the inner surface of the steel tube and the ceramic, and infiltration metal leaks through. These problems increase cost and delay production by requiring hand machining of the final mold.
The molding of parts from metal or ceramic powder may be also be accomplished by the xe2x80x9cpowder-injection moldingxe2x80x9d process. Thereby, a binder/powder wet paste is formulated in which the binder serves as a lubricant. The mixture is forced into the mold under pressure and the binder subsequently removed by heating to form a porous green preform. The preform is heated to its sintering temperature and an infiltration metal is added thereafter.
While this process is an improvement over the first generation xe2x80x9cpress and sinter metallurgyxe2x80x9d process, the resulting parts shrink considerably when heated to remove the binder from the preform, and there are limitations on the size of parts that can be fabricated by this process. Thus, these powders are not applicable to the reproducing the intricate patterns that may be formed by RP technology. Examples of powder-injection molding processes and powders include the following publications:
U.S. Pat. No. 4,906,424, issued to Hughes et al. on Mar. 6, 1990, discloses a method for injection molding ceramic or metallic powders by use of a binder. The ceramic powder may have a multi modal particle size distribution such that smaller diameter particles are provided to fill the interstices between larger particles. The primary binder material is a polymerized monomer or mixture of monomers that may be polymerized thermally, radiatively, or catalytically. The polymerized monomers may include various polyols. Suitable dispersants or surfactants such as oleic acid and stearic acid may be included in the binder as processing aids.
The mixture of binder and ceramic or metallic material may be injection molded. The mold temperature is maintained at about 50xc2x0 C. to about 200xc2x0 C. to initiate polymerization. The binder may be burned off by heating the preform to a temperature below about 700xc2x0 C. Finally, the molded article is sintered at a temperature ranging from approximately 700xc2x0 C. to 2200xc2x0 C. to obtain the final product.
The aforementioned prior art process has two significant drawbacks. First, forming intricate or irregularly shaped parts is often difficult because the powder compositions have inadequate flow properties that lead to density variations within the parts. In addition, the aforementioned process often produces shrinkage and distortion of parts during heating phases, since binders are essentially totally removed without modifying the connecting structure between the powder particles.
U.S. Pat. No. 5,328,657, issued Jul. 12, 1994 to Kamel et al. discloses a method of molding metal particles by producing a flowable mixture in which the binder chemically reacts with the metal particles. The flowable mixture is transferred to a mold before the chemical reaction between the metal particles and the polyorganic acid proceeds as far as to substantially increase the viscosity of the flowable mixture. The proportion of the metal powder to the binder is about 40 to about 60 volume percent. As stated in the patent, even when the green preform has a porosity greater that 50% before sintering, there is only about a 2 to 3% density reduction. However, forming a green preform that is almost fully dense for ensuring against density variations and providing greater detail is preferable.
Thus, a satisfactory method for using rapid prototyping still does not exist. Accordingly, there is still a need for a method to advantageously incorporate the rapid prototype (RP) technology into a rapid tooling method satisfactorily in terms of cost and turn around time.
According to the present invention, the above and other deficiencies of the prior art are alleviated or eliminated by a method for molding metal particles, ceramic particles or mixtures thereof and compositions therefor. According to the present invention, a method of molding particles to form molded articles, comprises the steps of:
a. mixing at least about 95% by weight sinterable metal particles, sinterable ceramic particles or mixtures thereof with at least about 0.5 wt. % to about 5.0 wt. % binder to form a powder mixture;
b. casting the mixture around a pattern wherein the pattern comprises a meltable, soluble or decomposable substance;
c. applying pressure sufficient to compact the mixture to form a preform;
d. removing the pattern by melting, dissolving or decomposing;
e. heating the preform at a sintering temperature sufficient to sinter the particles and form a molded article.
According to a preferred method, the pattern comprises a polymer which is formed by the method of RP technology and step (d) comprises removing the pattern by either heating the preform to a melting temperature sufficient to cause the pattern to melt and flow into a recycling container or subjecting the preform to a solvent to dissolve the pattern. Preferably, the melting temperature is in the range of from about 110xc2x0 C. to about 115xc2x0 C., or in the range from about 130xc2x0 C. to about 135xc2x0 C. such that water wave figures are formed on the surface of the mold for forming a decorative design.
According to other preferred embodiments, the preform is first heated to a drying temperature to thoroughly dry the particles, then either heated to the melting temperature to remove the pattern or treated with at least one solvent to dissolve the pattern, for example when the pattern comprises a polymer, and then the preform is removed from the dryer for cooling. Thereafter, the preform is heated to a vaporizing temperature to vaporize the binder in a furnace, then heated in stages to higher sintering and/or infiltration temperatures in the presence of a protective gas to sinter and infiltrate the metal particles with an infiltration metal.
According to the another preferred embodiment, the sintering and infiltration of green compacts are carried out simultaneously under a properly controlled temperature, whereby which both shrinkage and distortion of molded article can be reduced or eliminated.
According to a second aspect of the present invention, the above and other deficiencies of the prior art are alleviated or eliminated by providing a composition for molding metal particles, ceramic particles or mixtures thereof in which the metal and/or ceramic particles are combined with a small amount of binder. Preferably, the metal and/or ceramic powders are present in an amount of about 95% by weight to about 99.5% by weight and the binder is present in an amount of about 0.5 wt. % to about 5 wt. % to form a powder mixture.
Accordingly, the present invention provides for a method of preparing sintered articles without the aforementioned problems. Further, by using the method of the present invention, metal, ceramic or ceramic-metal parts and molds can be manufactured from RP models and the polymeric material used for preparing the pattern can be easily recycled.